Amplifier circuits



. July 29,1947. H. l.. MANSFORD 2,424,893

AMPLIFIER CIRCUITS l Flled Aug 22 1945 *x -vWs/L Q Patented July 29,1947 u ,n

UNITED STATES 4- PATENT OFFICE AMPLIFIER crncm'rs v Hugh Lyon Mansford,ckenh'am, Imglanel,as-A

signor to Electric &Musical1ndustries Limited, Hayes, England,lAajcompany-oi -Great k"Britain Application August 22, 1945,;se14a1No.612,033

In Great Britain April 2 4, 1944 ,section 1, P'ubiicLaw cemnugust s,194s.

`Patentexpres April 24, V1964 The present invention relates toamplifiers and more particularly to ampliers operating down to lovvfrequencies including direct current(D.'G.').

In known ampliers the tubes of the ampliiier are usually coupled incascade so that the anode circuit of each tube is in parallel with thev"Source of anode voltage.v When low frequencies, including D. C., areto be amplified, the couplingsbetween successive tubes are usually suchas will pass D. C. and generally include means for providing grid biasso as to avoid impressing the positive voltage of the anode of one tubeupon the grid of the next tube. v j

Known arrangements of this kind suffer `from the disadvantage thatinstability can ariseas 1va result of back-coupling via the impedance ofthe source of anode voltage.

It has also been proposed to connect the -individual tubes of amulti-tube amplifier with their anode-cathode paths in series across thesource of anode voltage. This reduces the instability due to couplingvia the impedance of the :sourcev of anode voltage but has the greatdisadvantage that because substantially the same current 'mus-t flowthrough all the series-connected tubes,-fthe change of current caused bythe signal applied to the control electrode of the rst tube of vtheseries is the same as the change of current in the rest ofthe tubes. Thearrangement is thus equivalent to a single tubeamplier with an increasedanode impedance andvvith the output taken from only part of this anodeimpedance and therefore gives substantially no more amplication than canbe obtained from a single tube-amplifier.

The object of the present inventicn'istoprovide an amplifier in whichthedisadvantages above referred to are substantially avoided.

According to the present invention thereislprovided an amplifiercomprising a source of anode voltage, a vplurality of pairs vofthermionictubes' the tubes of each of said pairs having their cathodesconnected to a common point-and theiranodes connected to a common point,means associated with each of said pairs for Vvarying lthe voltage ofthe control electrodes of each tube of each pair diiierentially'so as tocauseequal and opposite changes in the anode currents 'of' both tubes ofeach pair, whereby in operation vthe total anode curr-ent of each ofsaid pairs remains7 substantially unchanged, and connections ybe tweenthe common points to whichthe-cathodes comms. (01179-171) f Preferably,said means for varying the voltages of the control electrodes of each ofsaid pairs other than the lowermost pair comprises impedances includedbetween the anodes of the tubes of the preceding pair and the commonpoint to which said anodes are connected and connections between saidanodes and said control electrodes respectively. and if desired aresistance may be connected in shunt with at least one of said precedingpairs so that the total anode current of at least one pair is greaterthan that of a preceding pair.

In order that the said invention may be clearly understood and readilycarried-into effect it will now be more fully described with referencetothe accompanying drawings, in which:

IFig.` 1 illustrates a simplied circuit according tothe invention, and

' `Fig. 2 is a circuit diagram illustrating a practical embodiment ofthe invention.

l Referring now to Fig. 1, reference numerals l, la indicate triodertubes of the lowerrnost pair having cathodes, control grids and anodesrespectively numbered 2, 2a, 3, 3a, 4, ldat. Cathodes `2, 2a aredirectlyconnected to a common point, namely', to thepositive -pole of -a sourceof grid bias 6, the negative pole of which is connected to the negativepole of the source of anode voltage (not shown). The anodes 4, 4a arealso connected to a common point via impedances 5, 5a respectively,

Signals to be `amplified are applied to grids 3, 3a, in push-pull by Wayci input terminals I and I. The changes of the anode currents of tubesl, la will herefore be equal and opposite and the total anode currentpassed by these tubes in operation will beconstant. A further pair oftubes, indicated in'Fig. 1 by numerals 1, 1a, have cathodes 8,8m controlgrids 9, 9a anodes I, Ilia, respectively, their cathodes 8, 8a, beingdirectly connected to the common point to which anodes 4v, 4a of thepreceding pair of valves are connected and their anodes I0, Illa beingconnected to a further common point via impedances Il, lla. Control.grids` 9, 9a are directly connected to anodes 4, Aa., respectively anddue to the voltage drop across impedances 1.5, 5a, tubes 1, 1a aretherefore driveninpush-pull when tubes Vl la are so driven. AS n :thecase of tubes I, la., the-total current ovving into the commonpoint towhich anodes t0, 40a are connected ,is constant due to the fact that thecurrent changes `in the anode circuits ofjtubes 1, 'la are equal andopposite and the presence of tubes 11,' la in series with the anode cur-,polarising voltage.

are similar in principle but differ in detail in one or two respects.Thus, the tubes in Fig. 2 are shown as pentode valves having theirsuppressor grids connected to their cathodes and having the screening4grids `of each pair connected together and'viaafeed resistance to asource of suitable As shown, the screening grids of tubes I, Ia are fedvia resistance I1 conimpedances I6, IEa is taken to the positive pole ofthe source of anode voltage so that the same constant current flowsthrough the anode circuits of each pair of tubes I2, I2a, 1, 1a, and I,VIa in series. It will be appreciated that further pairs of tubes drivenin push-pull can if desired be connected in series with the three pairsshown in Fig. 1.

In the circuits shown in Fig. 1, each of the tubes operates as anamplifier for push-pull signals, and, due to the absence of couplingcapacities, the ampliiication is maintained down to the lowestfrequencies including, D. C. Further, under conditions of proper balancethere will be no variations in the current supplied by the source ofanode current due to signals and as there is no feedback path wherebyvoltages developed across the impedance of the source of anode voltageby small out-of-balance signal currents which may arise in practice canbe fed back to the early tubes of the amplier, there is vno likelihoodof instability from this cause as is the case with the normal cascadeamplifier operating down to the lowermost frequencies. A further featureof the-circuit arrangement illustrated in Fig. 1 is that the amplier isrelatively insensitive to signals which are applied to the tubes of eachpair in the same sense. In so far as signals of this kind are concerned,each pair of tubes may be regarded as being connected in parallel andbehaving in a similar manner to previously proposed series connectedamplifier arrangements hereinbefore referred to which are equivalent tosingle tube amplifiers with tapped down anode impedances. This featureis` advantageous in that out-of-balance currents tend to be suppressedand slight ripple on the source of anode voltage does not give rise toany troublesome effects.

It will be appreciated that, provided each lpair of tubes I, Ia, 1, 1a,I2, I2a, etc., are driven in push-pull, the relative phase in I which,for example, tubes I and 1, are driven is immaterial.4 Thus, controlgrid S can, if desired, be connected to anode 4a and control grid 9aconnected to anode 4 instead of as shown in the gure. Further, if it isnot desired to amplify the lowest frequencies, direct connectionsbetween the control grids and the anodes of the tubes of the precedingstage need not be used and may be replaced, for example, by resistancecouplings or transformer couplings. n

As the later stages of the amplifier have to handle 'larger signalcurrent variations than the preceding stages of the amplifier Vit isinconvenient to have all the ypairs of tubes of the amplifier Vpassingthe same total anode current. Reference will now be made to Fig. 2 whichillustrates a, circuit arrangementV in which this lmitation is avoidedand some other modications are introduced-for convenience. In Fig. 2elements corresponding to elements of Fig. l. are given the samereference numerals and it will be seen that the arrangements: of Figs. 1and 2 nected to cathodes 8, 8a, the screening grids of ytubes 1, 1a arefed via resistance I8 connected to cathodes I3, I3a and the screeninggrids of tubes I2, I2u. are fed via resistance I9 connected to thepositive pole of the source of anode voltage.- Battery 6 of Fig. 1 hasbeen replaced by the resistance 20 common to the cathode circuits ofSLVtubes I, Ia,`and a potentiometer comprising series connectedresistances 2| and 22 across the source of anode voltage is provided foradjusting the-bias on grids 3, 3a, grid 3a being connected through leakresistance 23 to the junction of resistances 2I and 22, the inputcircuit, notshown, providing a circuit feeding the bias Y to controlgrid 3.

The use of tubes of different current carrying capacities for thedifferent stages of the amplier is made possible by means of shuntingresistances 24 Vand 25. Resistance 25 is connected between cathode I3,VI3a and the negative pole of the source of anode voltage and has a valuesuch that it-,passes the difference between the total anode and screencurrent passed by valves I2, I2a. and the total anode and screen currentpassed by tubes 1, 1a. Similarly, resistance 24 connected betweencathodes 8, 8a. and the negative pole of the source of anode voltagepasses the difference between the total anode and screen current drawnby tubes 1, 1a. and the total anode and screen current drawn by tubes I,Ia. It will be appreciated that although these shunting resistances areillustrated as resistances they can if desired take the vform of furtherapparatus including tubes such as, for example, further amplifiersaccording to the present invention, providing that these latter.arearranged to operate with a constant total anode current ofappropriate value.

L* `lit may sometimes happen that the negative bias applied, forexample, to control grid 9 by the voltage drop across resistance 5 isunsuitable and this may be adjusted by the introduction of a suitablychosen resistance between control grid 9U and ranode 4 and an additionalresistance of suitable value between control grid 9 and a point ofconstant positive voltage such as cathodes I3, I3a`or the positive poleof the source of anode voltage. Alternatively, or in addition,resistances may .beintroduced between the common point of, for example,resistances 5, 5a and cathodes 8, 8a.

Provided that resistance 20 is suitably chosen, one ,of the -grids 3,3a, may be earthed and the signal appliedV unbalanced to the othercontrol grid, the 4signalrvoltage developed across resistancelnrservingto vary the cathode potential of the otherV tube and thereby driving itin antiphase. In vvsome cases, the resistance 20 may taire theform ofthe anode cathode path of a high impedance tube such as a pentode andthe total current'v taken by tubes I, Ia can then be adjusted by`achange of the bias on the control grid and thefscreening grid of thishigh impedance tube and, if desired, the output of the amplier can becontrolled or lreduced to zero by application of asuitable negative biasto this tube. By adding further'tubesy 3. and 3a' arranged similarly totubes 3, V3a,l and a further tube serving as a common resistance, saidtubes 3' and 3a' sharing anode resistances 5 and 5a with tubes 3 and 3arespectively, a convenient switching circuit is provided which enablesthe signals applied to tubes 3, 3a or 3', 3a' to be alternately fed tothe output of the ampliiier as the tubes in the common cathode circuitsare alternately rendered conducting and non-conducting by switchingsignals.

Although the invention has been described with particular reference toamplification operating down to very low frequencies including D. C. itwill be understood that it may be used With advantage even when it isnot desired to amplify down to such low frequencies.

What I claim is:

1. An amplifier comprising a source of anode voltage, a plurality ofpairs of thermionic tubes, the tubes of each of said pairs having theircathodes connected to a common point and their .anodes connected to acommon point, means associated with each of said pairs for varying thevoltage of the control electrodes of each tube of each pairdifferentially so as to cause equal and opposite changes in the anodecurrents of both tubes of each pair whereby in operation the total anodecurrent of each of said pairs remains substantially unchanged,connections between the common points to which the cathodes and anodesof tubes of diierent pairs are connected such that the anode circuits ofsaid pairs are effectively 'connected in series across said source, apair of input terminals connected to the control electrodes of the firstpair of tubes, and a pair of output terminals connected to the anodes ofthe last pair of tubes.

2. An amplier according to claim 1 in which said means for varying thevoltages of the control electrodes of the tubes of each of said pairsother than the first pair comprises impedances included between theanodes of the tubes of the preceding pair and the common point to whichsaid anodes are connected and connection between said anodes and saidcontrol electrodes respectively.

3. An amplier according to claim 2 in which a resistance is connected inshunt with at least one of said preceding pairs so that the total anodecurrent of at least one pair is greater than that of a preceding pair.

4. An amplifying system for direct or alternating potentials, comprisingtwo sets of thermonic tubes each of which is provided at least with acathode, a grid and an anode, the tubes of each set being arranged inseries ywith the anode of one tube connected to the cathode of asucceeding tube, a connection between the cathodes of correspondingtubes of said sets, a source of anode potential supply having itspositive terminal connected to the anode of the last tube of each setand its negative terminal to the cathode of the rst tulbe of each set,means for supplying the potential to be amplied to the grid of the rsttube of each set, and means for deriving amplified output potential fromthe anodes of the last tubes of each set.

5. A system for amplifying direct or alternating potentials, comprisingtwo sets of thermionic tubes each having at least a cathode, a grid andan anode, the tubes of each set being arranged in series with the anodeof one tube connected to the cathode of a succeeding tube through aresistance, a source of anode potential supply having its positiveterminal connected through a resistance to the anode of the last tube ofeach set and its negative terminal connected through a resistance to thecathode of the first tube oi each set, a connection between the cathodesof corresponding tubes of said sets, a connection from the grid of eachtube except the first to the anode of the preceding tube, means forsupplying the potential to be amplified to the grid of 'the first tubeof each set, and means for deriving amplified output potential from theanodes of the last tubes of each set.

6. An amplifying system as dened in claim 5 wherein a resistance isconnected between the common cathode connection lof the rst tubes ofeach set and the common cathode connection of another pair ofcorresponding tubes.

HUGH LYON MANSFORD.

REFERENCES CITED The following references are of record in the Name DateMitchell et al Sept. '7, 1943 Number

